Methods and apparatus for testing earth formations composed of particles of various sizes

ABSTRACT

In the representative embodiments of the new and improved methods and apparatus disclosed herein for testing earth formations of differing compositions, fluid-admitting means carrying a selectively-sized filter of a unique design are selectively extended into sealing engagement with a potentiallyproducible earth formation and operated so as to establish communication with the isolated formation without the fluidadmitting means being plugged with mudcake from the formation wall. Should, however, loose formation materials enter the fluidadmitting means as the testing is conducted, the filter is uniquely arranged to collect these loose materials and halt the further erosion of such materials from the formation wall so as to assure continued communication with the isolated formation.

United States Patent [1 1 Bell [451 Feb. 11, 1975 METHODS AND APPARATUSFOR TESTING EARTH FORMATIONS COMPOSED OF PARTICLES OF VARIOUS SIZESWilliam T. Bell, Houston, Tex.

Schlumberger Technology Corporation, New York, NY.

Oct. 18, 1973 Inventor:

Assignee:

Filed:

App]. No.:

[52] US. Cl 73/155, 73/421 R, 166/264 [58] Field of Search 73/155, ll,421 R;

References Cited UNITED STATES PATENTS 6/1965 Briggs, Jr. 73/155 7/l972Hallmark 73/155 Primary Examiner- -Jerry W. Myracle Attorney, Agent, orFirm-Ernest R. Archambeau, Jr.; William R. ShermamStewart F. :Moore Int.Cl E2lb 49/04 [57] ABSTRACT lect these loose materials and halt thefurther erosion of such materials from the formation wall so as toassure continued communication with the isolated formation. v

34 Claims, 11 Drawing Figures 54 LI I 55 7 55 PATENIEU E U S M 3.864.970

sum nor 7 METHODS AND APPARATUS FOR TESTING EARTH FORMATIONS COMPOSED OFPARTICLES OF VARIOUS SIZES Until very recently, the so-called wirelineformation testers, which have been most successful in commercial servicehave, for the large part, been limited to attempting only a single testor, at best, two tests of selected earth formations. Generally, thesuccess of these tests has depended to some extent upon knowing inadvance the general character of the particular formations which were tobe tested so that the tester could be equipped as required to test aformation of a given nature.

For example, where the formations to be tested were considered to befairly competent and, therefore, not easily eroded, testers such as thatshown in US. Pat. No. 3,01 1,554 have been highly effective. On theother hand, in those situations where tests were to be conducted infairly incompetent or unconsolidated formations, it has usually been thepractice to use new and improved testers such as those shown in US. Pat.No. 3,352,361, US. Pat. No. 3,530,933, US. Pat. No. 3,565,169 or US.Pat. No. 3,653,435. As fully described in these last-mentioned patents,each of those testing tools employs a tubular sampling member which iscooperatively associated with a filtering medium having fluid openingsof a selected, but uniform, size for preventing the unwanted entrance ofunconsolidated formation materials into the testing tool. Thus, exceptfor dual-purpose tools such as that shown in U.S. Pat.

No. 3,261,402, these typical formation testing tools have been mostsuccessful in making tests in formations which are known in advanceeither to be fairly competent or to be relatively unconsolidated.Moreover, since all of these prior-art testers are operated only onceduring a single trip into a well bore, it has been customary to simplyselect in advance the particular size of filter medium believed to bebest suited for a given testing operation.

One of the most'significant advances in the formation-testing art hasbeen the recent introduction into commercial service of the new andimproved repetitively-operable testers such as fully described in US.Pat. No. 3,780,575. As disclosed there, these tools are capable ofrepetitively taking any number of pressure measurements from variousformations as well as collecting at least two fluid samples during asingle trip in a given well bore.

Although these new and improved testers have been quite successful,there are situations where the performance of these testers issignificantly affected since no one filtering medium is capable ofoperating efficiently with every type of earth formation. For instance,if the tester is equipped with a particular filter which is best suitedfor stopping exceptionally-fine formation materials, the flow rate forthis tester will be materially limited where a fairly-competentformation is being tested. More importantly, in situations like this, itis not at all uncommon for the filter to be quickly plugged by themudcake which usually lines the borehole wall adjacent to apotentially-producible formation. Thus, a test under these conditionswill often be inconclusive, if not misleading, since it will not beknown for sure whether the formation is truly unproductive or if thefilter was simply plugged at the outset of the test. On the other hand,if the tester is then using a filter designed for filtering out onlyfairly-large loose formation materials, there will often be an excessiveinduction of very-fine formation materials into the tool where the toolis testing a highly-unconsolidated formation. This will, of course,frequently result in a continued erosion of the formation wall aroundthe sealing pad so that communication with the formation is quicklylost. This also causes an incomplete or inconclusive test.

It will be recognized, of course, that is wholly impractical to changethe filter in a repetitively-operable tool of this type between tests ofdifferent types of formations in a given borehole. Moreover, there is noassurance that the character of various formations traversed by a givenborehole can be reliably determined in advance.

Accordingly, it is an object of the present invention to provide new andimproved formation-testing methods and apparatus for reliably obtainingmultiple measurements of one or more fluid or formation characteristicsas well as for selectively collecting one or more samples of connatefluids, if desired, from different earth formations of any charactereven where these formations vary in their compositions and competency.

This and other objects of the present invention are attained byproviding formation-testing apparatus having fluid-admitting meansadapted for selective movement into sealing engagement with apotentiallyproducible earth formation to isolate a portion thereof fromthe borehole fluids. ln practicing the methods of the present inventionfor testing a formation, mudcake is first inducted into thefluid-admitting means by way of a first filtering passage selectivelysized to readily pass such plugging materials. Then, should incompetentformation materials be drawn into the fluidadmitting means, thesematerials are collected so as to halt their further erosion from theborehole wall by quickly blocking at least substantial flow through thisfirst passage as well as thereafter directing the flow of connate fluidsthrough a second filter passage selectively sized to be smaller than theformation materials. In the new and improved apparatus of the presentinvention, the fluid-admitting means are provided with filtering meanshaving one or more enlarged filter passages sized to easily pass largeplugging materials such as mudcake particles and one or more reducedfilter passages which are sized to screen or retain formation particlesof a selected size. In this manner, when the fluid-admitting means areinitially placed into communication with an isolated earth formation,mudcake lining the formation wall will be passed through the enlargedfilter passages thereby leaving the inlet face of the filtering meansfree of such plugging materials. Thereafter, should loose formationmaterials be inducted into the fluid-admitting means, they will becollected in a compact mass along the inlet face of the filtering meansso as to effectively block at least significant flow through theenlarged filter passages and direct the flow of producible connatefluids through the reduced filter passages.

The novel features of the present invention are set forth withparticularity in the appended claims. The invention, together withfurther objects and advantages thereof, may be best understood by way ofthe following descriptions of the new and improved methods of thepresent invention as well as various embodiments of exemplary apparatusemploying the principles of the invention as illustrated in theaccompanying drawings, in which:

Flg. 1 depicts the surface and downhole portions of one embodiment offormation-testing apparatus including new and improved fluid-admittingmeans incorporating the principles of the present invention;

FIG. 2 is an enlarged view ofa preferred embodiment of the new andimproved fluid-admitting means shown in FIG. 1;

FIGs. 3A and 3B together show a somewhatschematic representation of theformation-testing tool illustrated in FIG. 1 as the tool will appear inits initial operating position in readiness for practicing the new andimproved methods of the invention;

FIGS. 4, 5, 6A and 68 respectively depict the successive positions ofvarious components of the testing tool shown in FIGS. 3A and 38 duringthe course of a typical testing and sampling operation to illustrate themethods as well as the operation of the new and improved fluid-admittingmeans of the present invention; and

FIGS. 7-9 schematically illustrate the practice of the methods of thepresent invention by the new and improved fluid-admitting means withdifferent types of earth formations as well as depict alternativeembodiments of filter members which may be employed therewith to achievethe objects of the present invention.

Turning now to FIG. 1 a preferred embodiment of new and improvedfluid-admitting means 10 incorporating the principles of the presentinvention is shown on a formation-testing tool 11 as this tool willappear during the course of a typical measuring and sampling operationin a well bore such as a borehole 12 penetrating one or more earthformations as at 13 and 14. As illustrated, the tool 11 is suspended inthe borehole 12 from the lower end of a typical multiconductor cable 15that is spooled in the usual fashion on a suitable winch (not shown) atthe surface and coupled to the surface portion of a tool-control system16 as well as typical recording-and-indicating apparatus 17 and a powersupply 18. In its preferred embodiment, the tool 11 includes anelongated body 19 which encloses the downhole portion of thetool-control system 16 and carries a selectively-extendibletool-anchoring member 20 arranged on one or more piston actuators, as at21, for movement from the opposite side of the body from the new andimproved fluid-admitting means 10 as well as one or morefluid-collecting chambers 22 and 23 which are tandemly coupled to thelower end of the tool body 19.

As is explained in greater detail in U.S. Pat. No. 3,780,575 which isincorporated by reference herein, the depicted formation-testing tool 11and its control system 16 are cooperatively arranged so that, uponcommand from the surface, the tool can be selectively placed in any oneor more of five selected operating positions. As will be subsequentlydescribed briefly, the control system 16 will function either tosuccessively place the tool 11 in one or more of these positions or elseto selectively cycle the tool between various ones of these operatingpositions. These five operating positions are simply achieved byselectively moving suitable control switches, as schematicallyrepresented at 24 and 25, included in the surface portion of the controlsystem 16 to various switching positions, as at 26-31, so as toselectively apply power to different conductors 32-38 in the cable l5.

The new and improved fluid-admitting means 10 of the present inventionare cooperatively arranged for selectively sealing-off or isolatingselected portions of the wall of the borehole l2; and, once a selectedportion of the borehole wall is packed-off or isolated from the boreholefluids, establishing pressure or fluid communication with the adjacentearth formation, as at 13. In the preferred embodiment depicted in FIG.2, the fluid-admitting means 10 include an elastomeric annular sealingpad 39 mounted on the forward face of an upright support member or plate40 that is coupled to a longitudinally-spaced pair of laterally-movablepiston actuators, as at 41, which are similar to the actuators 21 andare arranged transversely on the tool body 19 for moving the sealing padback and forth in relation to the forward side of the tool body.Accordingly, as the control system 16 selectively supplies a pressuredhydraulic fluid to the piston actuators 4], the sealing pad 39 will bemoved laterally between a retracted position adjacent to the forwardside of the tool body 19 and an advanced or forwardly-extended position.

By arranging the annular sealing member 39 on the opposite side of thetool body 19 from the toolanchoring member 20 (FIG. 1), the simultaneousextension of these two wall-engaging members will, of course, beeffective for urging the sealing pad into sealing engagement with theadjacent wall of the borehole 12 as well as for anchoring the tool 11.It should. however, be appreciated that the tool-anchoring member 20would not be needed if the effective stroke of the piston actuators 41is sufflcient for assuring that the sealing pad 39 can be extended intofirm sealing engagement with one wall of the borehole 12 with the rearof the tool body 19 securely anchored against the opposite wall of theborehole. Conversely, the piston actuators 21 could be similarly omittedwhere the extension of the tool-anchoring member 20 alone would beeffective for moving the front side of the tool body 19 forwardly towardone wall of the borehole 12 so as to place the sealing pad 39 into firmsealing engagement therewith. However, in the preferred embodiment ofthe formation-testing tool 11, both the toolanchoring member 20 and thefluid-admitting means 10 are arranged to be simultaneously extended toenable the tool to be operated in boreholes of substantial diameter.This preferred design of the tool 11, of course, keeps the overallstroke of the piston actuators 21 and 41 to a minimum so as to reducethe overall diameter of the tool body 19.

To conduct connate fluids into the testing tool 11, the fluid-admittingmeans 10 of the present invention further include an enlarged tubularmember 42 having an open forward portion coaxially disposed within theannular sealing pad 39 and a closed rear portion which is slidablymounted within a larger tubular member 43 secured to the rear face ofthe plate 40 and extended rearwardly therefrom. By arranging the nose ofthe tubular fluid-admitting member 42 to normally protrude a shortdistance ahead of the forward face ofthe sealing pad 39, extension ofthe fluid-admitting means 10 will engage the forward end of thefluid-admitting member with the adjacent surface of the wall of theborehole 12 just before the annular sealing pad is also forcedthereagainst for isolating that portion of the borehole wall as well asthe nose of the fluid-admitting member from the borehole fluids. Thesignificance of this sequence of engagement will be subsequentlyexplained. To selectively move the tubular fluid-admitting member 42 inrelation to the enlarged outer member 43, the smaller tubular member isslidably disposed within the outer tubular member and fluidly sealed inrelation thereto as by sealing members 44 and 45 on inwardly-enlargedend portions 46 and 47 of the outer member and a sealing member 48 onthe enlarged-diameter intermediate portion 49 of the inner member.

Accordingly, it will be appreciated that by virtue of the sealingmembers 44, 45 and 48, enclosed piston chambers 50 and 51 are definedwithin the outer tubular member 43 and on opposite sides of theoutwardlyenlarged portion 49 of the inner tubular member 42 which, ofcourse, functions as a piston member. Thus, by applying an increasedhydraulic pressure in the rearward chamber 50, the fluid-admittingmember 42 will be moved forwardly in relation to the outer tubularmember 43 as well as to the sealing pad 39. Conversely, upon theapplication of an increased hydraulic pressure to the forward pistonchamber 51, the fluid-admitting member 42 will be retracted in relationto the outer member 43 and the sealing pad 39.

Pressure or fluid communication with the new and improvedfluid-admitting means of the present invention is preferably controlledby means such as a generally-cylindrical valve member 52 which iscoaxially disposed within the fluid-admitting member 42 andcooperatively arranged for axial movement therein between a retracted oropen position and the illustrated advanced or closed position where theenlarged forward end 53 of the valve member is substantially, if notaltogether, sealingly engaged with the forwardmost interior portion ofthe fluid-admitting member. To support the valve member 52, the rearwardportion of the valve member is axially hollowed, as at 54, and coaxiallydisposed over a tubular member 55 projecting forwardly from thetransverse wall 56 closing the rear end of the fluid-admitting member42. The axial bore 54 is reduced and extended forwardly along the valvemember 52 to a termination with one or more transverse fluid passages 57in the forward portion of the valve member just behind its enlarged head53.

To provide actuating means for selectively moving the valve member 52 inrelation to the fluid-admitting member 42, the rearward portion of thevalve member is enlarged, as at. 58, and outer and inner sealing members59 and 60 are coaxially disposed thereon and respectively sealinglyengaged with the interior of the fluid-admitting member and theexteriorof the forwardly-extending tubular member 55. A sealing member61 mounted around the intermediate portion of the valve member 52 andsealingly engaged with the interior wall of the adjacent portion of thefluid-admitting member 42 fluidly seals the valve member in relation tothe fluid-admitting member. Accordingly, it will be appreciated that byincreasing the hydraulic pressure in the enlarged piston chamber 62defined to the rear of the enlarged valve portion 58 which serves as apiston member, the valve member 52 will be moved forwardly in relationto the fluid-admitting member 42. Conversely, upon application of anincreased hydraulic pressure to the forward piston chamber 63 definedbetween the sealing members 59 and 61, the valve member 52 will be movedrearwardly along the forwardlyprojecting tubular member 55 so as toretract the valve member in relation to the fluid-admitting member 42.

As previously discussed, it will, of course, be appreciated that manyearth formations, as at 13, are relatively unconsolidated and are,therefore, readily eroded by the withdrawal of connate fluids. Thus, toprevent any significant erosion of such unconsolidated formationmaterials, the fluid-admitting member 42 is arranged to define aninternal annular space 64 and a flow passage 65 in the forward portionof the fluid-admitting member. As will subsequently be described ingreater detail by reference to FIGS. 7-9, the objects of the presentinvention are preferably attained by coaxially mounting a tubular filtermember 66 (or 66) with slits or apertures therein ofa unique arrangementin the nose of the fluid-admitting member 42 so as to cover the annularspace 64. In this manner, when the valve member 52 is retracted from itsextended position inside of the filter. formation fluids will becompelled to pass through the now-exposed filter member 66 ahead of theenlarged head 53, into the annular space 64, and then through the fluidpassage 65 into the fluid passage 57 and the tubular member 55. Thus, asthe valve member 52 is retracted, should loose or unconsolidatedformation materials be eroded from a formation as connate fluids arewithdrawn therefrom, the materials will be stopped by theuniquely-arranged filter 66 ahead of the en- I larged head 53 of thevalve member thereby quickly forming a permeable barrier to prevent thecontinued erosion of loose formation materials once the valve memberhalts.

Turning now to FIGS. 3A and 3B, the new and improved fluid-admittingmeans 10 as well as the entire downhole portion of the control system16, the toolanchoring member 20, and the fluid-collecting chambers 22and 23 are schematically illustrated with their several elements orcomponents depicted as they will respectively be arranged when the tool11 is fully retracted and the control switches 24 and 25 are in theirfirst or off operating positions 26 (FIG. 1). Since the aforementionedU.S. Pat. No. 3,780,575 fully describes the control system 16 andvarious components of the tool 11, it is believed adequate to simplycover only the major aspects of this system.

A sample or flow line 67 is cooperatively arranged in theformation-testing tool 11 and has one end coupled, as by a flexibleconduit 68, to the fluid-admitting means 10 and its other end terminatedin a pair of branch conduits 69 and 70 respectively coupled to thefluidcollecting chambers'22 and 23. To control fluid communicationbetween the new and improved fluidadmitting means 10 and thefluid-collecting chambers 22 and 23, normally-closed flow-control valves71-73 of a similar or identical design are arranged respectively in theflow line 67 and in the branch conduits 69 and 70 leading to the samplechambers. For reasons which will subsequently be described, anormally-open control valve 74 which is preferably similar to thenormally-closed control valves 71-73 is cooperatively arranged in abranch conduit 75 for selectively controlling communication between theborehole fluids exterior of the tool 11 and the upper portion of theflow line 67 and the flexible conduit 68 extending between the flow-linecontrol valve 71 and the new and improved fluid-admitting means 10.

As illustrated, the normally-open control valve 74, for example, isoperated by a typical pressureresponsive actuator 76 which is arrangedto close the valve in response to an actuating pressure of at least apredetermined magnitude. As fully described in the aforementioned U.S.Pat. No. 3,780,575, a spring biasing the control valve 74 to its openposition is cooperatively arranged to establish the magnitude of thepressure required to close the valve. Furthermore, the normally-closedcontrol valves 71-73 are preferably similar to the control valve 74except that they are respectively operated by pressure-responsiveactuators 77-79 selectively arranged to open these valves in response topressures of different predetermined magnitudes.

in the particular embodiment of the testing tool 11 shown in FIGS. 3Aand 38, a branch conduit 80 is coupled to the flow line 67 at aconvenient location between the sample-chamber control valves 72 and 73and the flow-line control valve 71, with this branch conduit beingterminated at an expansion chamber 81 of a predetermined volume. Areduced-diameter displacement piston 82 is operatively mounted in thechamber 81 and arranged to be moved between selected upper and lowerpositions therein by a typical piston actuator shown generally at 83.Accordingly, it will be appreciated that upon movement of thedisplacement piston 82 from its lower position as illustrated in FIG. 3Ato an elevated or upper position, the combined volume of whatever fluidsthat are then contained in the branch conduit 80 as well as in thatportion of the flow line 67 between the flow-line control valve 71 andthe sample-chamber control valves 72 and 73 will be correspondinglyincreased.

As best seen in FIG. 3A, the control system 16 further includes a pump84 that is coupled to a driving motor 85 and cooperatively arranged forpumping a suitable hydraulic fluid such as oil or the like from areservoir 86 into a discharge or outlet line 87. Since the tool 11 is tobe operated at extreme depths in boreholes, as at 12, which typicallycontain dirty and usually corrosive fluids, the reservoir 86 ispreferably arranged to totally immerse the pump 84 and the motor 85 inthe clean hydraulic fluid. The reservoir 86 is also provided with aspring-biased isolating piston 88 for maintaining the hydraulic fluid ata pressure about equal to the hydrostatic pressure at whatever depth thetool is then situated as well as accommodating volumetric changes in thehydraulic fluid which may occur under different borehole conditions. oneor more inlets, as at 89 and 90, are provided for returning hydraulicfluid from the control system 16 to the reservoir 86 during theoperation of the tool 11.

The fluid outlet line 87 is divided into two major branch lines whichare respectively designated as the set line 91 and the retract line 92.To control the admission of hydraulic fluid to the set and retract lines91 and 92, a pair of normally-closed solenoidactuated valves 93 and 94are cooperatively arranged to selectively admit hydraulic fluid to thetwo lines as the control switch 24 at the surface is selectivelypositioned; and a typical check valve 95 is arranged in the set line 91downstream of the control valve 93 for preventing the reverse flow ofthe hydraulic fluid whenever the pressure in the set line is greaterthan that then existing in the fluid outlet line 87. Typical pressureswitches 96-98 are cooperatively arranged in the set and retract lines91 and 92 for selectively starting and stopping the pump 84 as requiredto maintain the line pressure within a selected operating range. Sincethe pump 84 is preferably a positive-displacement type to achieve arapid predictable rise in the operating pressures in the set and retractlines 91 and 92, each time the pump is to be started the control system16 also functions to temporarily open the control valve 94 (if it is notalready open) as well as a third normally-closed solenoid-actuated valve99 for bypassing hydraulic fluid directly from the output line 87 to thereservoir 86 by way of the return line 89. Once the motor has reachedoperating speed, the bypass valve 99 will, of course, be reclosed andeither the set line control valve 93 or the retract line control valve94 will be selectively opened as required for that particularoperational phase of the tool".

Accordingly, it will be appreciated that the control system 16cooperates for selectively supplying pressured hydraulic fluid to theset and retract lines 91 and 92. Since the pressure switches 96 and 97respectively function only to limit the pressures in the set and retractlines to a selected maximum pressure range commensurate with the ratingof the pump 84, the control system 16 is further arranged tocooperatively regulate the pressure of the hydraulic fluid which isbeing supplied at various times to selected portions of the system.Although this regulation can be accomplished in different manners, it ispreferred to employ a number of pressure-actuated control valves such asthose shown schematically at 100-103 in FIGS. 3A and 3B for controllingthe hydraulic fluid in the control system 16. As shown in FlG. 3A, thehydraulic control valve 100, for example, includes a valve body 104having an enlarged upper portion carrying a downwardly-biased actuatingpiston 105 which is cooperatively coupled to a valve member 106 as by anupright stem 107 thereon which is slidably disposed in an axial bore 108in the piston. A spring 109 of selected strength is disposed in theaxial bore 108 for normally urging the valve member 106 into seatingengagement.

In its non-actuated position depicted in FIG. 3A, the control valve 100(as well as the valve 101) will, therefore, simply function as anormally-closed check valve. That is to say, in this operating position,hydraulic fluid can flow only in a reverse direction whenever thepressure at the valve outlet is sufficiently greater than the inletpressure to unseat the valve member 106 against the predeterminedclosing force imposed by the spring 109. On the other hand, whenever theactuating piston 105 is elevated by the application of hydraulicpressure thereto, opposed shoulders, as at 110, on the stem 107 and thepiston 105 will engage for unseating the valve member 106. As shown inFIGS. 3A and 38, it will be appreciated that the control valve 102 (aswell as the valve 103) is similar to the control valve 100 except thatin these first-mentioned control valves, the valve member, as at 111, ispreferably rigidly coupled to its associated actuating piston, as at112. Thus, the control valve 102 (as well as the valve 103) has noalternate checking action allowing reverse flow but is simply anormally-closed pressure-actuated valve for selectively controllingfluid communication between its inlet and outlet ports.

The set line 91 downstream of the check valve 95 is comprised of alow-pressure section 113 having one branch 114 coupled to the fluidinlet of the control valve 102 and another branch 115 which is coupledto the fluid inlet of the hydraulic control valve 100 to selectivelysupply hydraulic fluid to a high-pressure section 116 of the set linewhich is itself terminated at the fluid inlet of the hydraulic controlvalve 103. To regulate the supply of hydraulic fluid from thelow-pressure section 113 to the high-pressure section 116 of the setline 91, a pressure-communicating line 117 is coupled between thelow-pressure section and the control port of the hydraulic control valve100. Accordingly, so long as the pressure of the hydraulic fluid in thelowpressure section of the set line 91 remains below the predeterminedactuating pressure required to open the hydraulic control valve 100, thehigh-pressure section 116 will be isolated from the low-pressure section113. Conversely, once the hydraulic pressure in the lowpressure line 113reaches the predetermined actuating pressure of the valve 100, thehydraulic control valve will open to admit the hydraulic fluid into thehighpressure line 116.

The hydraulic control valves 102 and 103 are respectively arranged toselectively communicate the lowpressure and high-pressure sections 113and 116 of the set line 91 with the fluid reservoir 86. To accomplishthis, the control ports of the two hydraulic control valves 102 and 103are each connected to the retract line 92 by suitablepressure-communicating lines 118 and 119. Thus, whenever the pressure inthe retract line 92 reaches their respective predetermined actuatinglevels, the hydraulic control valves 102 and 103 will be respectivelyopened to selectively communicate the two sections 113 and 116 of theset line 91 with the reservoir 86 by way of the return line 89 coupledto the respective outlets of the two control valves.

As previously mentioned, in FIGS. 3A and 3B the tool 11 and thesub-surface portion of the control system 16 are depicted as theirseveral components will appear when the tool is retracted. At thispoint, the tool-anchoring member 20 and the sealing pad 39 arerespectively retracted against the tool body 19 to facilitate passage ofthe tool 11 into the borehole 12. To prepare the tool 11 for loweringinto the borehole 12, the switches 24 and 25 are moved from their firstor off positions 26 to their second or initialization positions 27. Atthis point, the hydraulic pump 84 is started to raise the pressure inthe retract line 92 to a selected maximum to be certain that the pad 39and the toolanchoring member 20 are fully retracted. At this time, thepressure-equalizing valve 74 is open and that portion of the flow line67 between the closed flow-line control valve 71 and the fluid-admittingmeans will be filled with borehole fluids as the tool 11 is beinglowered into the borehole 12.

When the tool 11 is at a selected operating depth, the switches 24 and25 are advanced to their third positions 28. Then, once the pump 84 hasreached its rated operating speed, the hydraulic pressure in the outputline 87 will rapidly rise to its selected maximum operating pressure asdetermined by the maximum or off setting of the pressure switch 96. Asthe pressure progressively rises, the control system 16 willsuccessively function at selected intermediate pressure levels forsequentially operating the several control valves 71-74 and 100-103 inan operating cycle such as the one described fully in the aforementionedUS. Pat. No. 3,780,575. It must, however, be recognized that theforthcoming particular operational sequence of the tool 11 asillustrated is not essential to either the practice of the methods ofthe present invention or the successful operation of the new andimproved fluidadmitting means 10. Those skilled in the art will,therefore, understand that the present invention can be practiced eitherwith different types of formationtesting tools or with differentarrangements of the tool 11 and the control system 16.

Turning now to FIG. 4, selected portions of the control system 16 andvarious components of the tool 11 are schematically represented toillustrate the operation of the illustrated embodiment of the tool atabout the time that the pressure in the hydraulic output line 87 reachesits lowermost intermediate pressure level. To facilitate anunderstanding of the operation of the tool 11 and the control system 16at this point in the operating cycle illustrated in the severaldrawings, only those components which are then operating are shown inFIG. 4.

At this time, since the control switch 24 (FIG. 1) is in its thirdposition 28, the solenoid valves 93 and 99 will be open; and, since thehydraulic pressure in the set line 91 has not yet reached the upperpressure limit as determined by the pressure switch 96, the pump motorwill still be operating. Since the hydraulic control valve 100 (notshown in FIG. 4) is closed, the highpressure section 116 of the set line91 will still be isolated from the low-pressure section 113.Simultaneously, the hydraulic fluid contained in the forward pressurechambers of the piston actuators 21 and 41 will be displaced (as shownby the arrows as at 120) to the retract line 92 and returned to thereservoir 86 by way of the open solenoid valve 99. These actions will,of course, cause the tool-anchoring member 20 as well as the sealing pad39 to be respectively extended in opposite lateral directions until eachhas moved into firm engagement with the opposite sides of the borehole12.

It will be noticed in FIG. 4 that hydraulic fluid will be admitted byway of branch hydraulic lines 121 and 122 to the enclosed annularchamber 50 to the rear of the enlarged-diameter portion 49 of thefluid-admitting member 42. At the same time, hydraulic fluid from thepiston chamber 51 ahead of the enlarged-diameter portion 49 will bedischarged by way of branch hydraulic lines 123 and 124 to the retractline 92 for progressively moving the fluid-admitting member 42 forwardlyin relation to the sealing member 39 until the nose of thefluid-admitting member engages the wall of the borehole 12 and thenhalts. The sealing pad 39 is then urged forwardly in relation to thenow-halted tubular member 42 until the pad sealingly engages theborehole wall for packing-off or isolating the isolated wall portionfrom the borehole fluids. In this manner, mudcake immediately ahead ofthe fluid-admitting member 42 will be displaced radially away from thenose of the fluid-admitting member so as to minimize the quantity ofunwanted mudcake which will subsequently be admitted into thefluid-admitting means 10. Those skilled in the art will appreciate thesignificance of this unique arrangement.

It should also be noted that although the pressured hydraulic fluid isalso admitted at this time into the forward piston chamber 63 betweenthe sealing members 59 and 61 on the valve member 52, the valve memberis temporarily prevented from moving rearwardly in relation to the innerand outer tubular members 42 and 55 inasmuch as the hydraulic controlvalve 101 (not shown in FIG. 4) is preferably still closed therebytemporarily trapping the hydraulic fluid in the rearward piston chamber62 to the rear of the valve member. The purpose of this delay in theretraction of the valve member 52 will be subsequently explained.

As also illustrated in FIG. 4, the hydraulic fluid in the low-pressuresection 113 of the set line 91 will also be directed by way of a branchhydraulic line 125 to the piston actuator 83. This will, of course,result in the displacement piston 82 being elevated as the hydraulicfluid from the piston actuator 83 is returned to the retract line 92 byway of a branch hydraulic conduit 126. As will be appreciated, elevationof the displacement piston 82 in the expansion chamber 81 will beeffective for significantly decreasing the pressure initially existingin the isolated portions of the branch line 80 and the flow line 67between the still-closed flow-line control valve 71 and the still-closedchamber control valves 72 and 73 (not shown in FIG. 4). The purpose ofthis pressure reduction will be subsequently explained.

Once the tool-anchoring member 20, the sealing pad 39 and thefluid-admitting member 42 have respectively reached their extendedpositions as illustrated in FIG. 4, it will be appreciated that thehydraulic pressure delivered by the pump 84 will again rise. Then, oncethe pressure in the output line 87 has reached its second intermediatelevel of operating pressure, the hydraulic control valve 101 will openin response to this pressure level to now discharge the hydraulic fluidpreviously trapped in the piston chamber 62 to the rear of the valvemember 52 back to the reservoir 86.

As illustrated in FIG. 5, once the hydraulic control valve 101 opens,the hydraulic fluid will be displaced from the rearward piston chamber62 by way of branch hydraulic lines 127, 128 and 124 to the retract line92 as pressured hydraulic fluid from the set line 91 surges into thepiston chamber 63 ahead of the enlargeddiameter portion 58 of the valvemember 52. This will, of course, cooperate to rapidly drive the valvemember 52 rearwardly in relation to the now-halted fluidadmitting member42 for establishing fluid or pressure communication between the isolatedportion of the earth formation 13 and the flow passages 54 and 57 in thevalve member by way of the filter member 66.

Although this is not fully illustrated in FIG. 5, it will be recalledfrom FIGS. 3A and 38 that the control valves 71-73 are initially closedto isolate the lower portion of the flow line 67 between these valves aswell as the branch line 80 leading to the pressure-reduction chamber 81.However, the flow-line pressureequalizing control valve 74 will still beopen at the time the hydraulic control valve 101 opens to retract thevalve member 52 as depicted in FIG. 5. Thus, as the valve member 52progressively uncovers the new and improved filtering member 66,borehole fluids at a pressure greater than that of any connate fluidswhich may be present in the isolated earth formation 13 will beintroduced into the upper portion of the flow line 67 and, by way of theflexible conduit member 68, into the rearward end of the tubular member55. As these highpressure borehole fluids pass into the annular space 64around the filtering member 66, they will be forcibly discharged (asshown by the arrows 129) from the forward end of the fluid-admittingmember 42 for washing away any plugging materials such as mudcake or thelike which may have become deposited on the internal surface of thefiltering member when it is first uncovered by the retraction of thevalve member 52. Thus, the particular embodiment of the control system16 illustrated in the drawings is operative for providing a momentaryoutward surge or reverse flow of borehole fluids for cleansing thefiltering member 66 of unwanted debris or the like before a sampling ortesting operation is commenced. This is, however, not essential to thesuccessful operation of the new and improved fluid-admitting means 10.

It will be appreciated that once the several components of theformation-testing tool 11 and the control system 16 have reached theirrespective positions as depicted in FIG. 5, the hydraulic pressure inthe output line 87 will again quickly increase to its next intermediatepressure level. Once the pump 84 has increased the hydraulic pressure inthe output line 87 to this next predetermined intermediate pressurelevel, the hydraulic control valve will selectively open as depicted inFIG. 6A. As seen there, opening of the hydraulic control valve 100 willbe effective for now supplying hydraulic fluid to the high-pressuresection 116 of the set line 91 and two branch conduits and 131 connectedthereto for successively closing the pressureequalizing valve 74 andthen opening the flow-line control valve 71.

In this manner, as respectively depicted by the several arrows at 132and 133, hydraulic fluid at a pressure representative of theintermediate operating level will be supplied by way of a typical checkvalve 134 to the upper portion of the actuator 76 of the normally-openpressure-equalizing valve 74 as fluid is exhausted from the lowerportion of the actuator by way of a conduit 135 coupled to the retractline 92. This will, of course, be effective for closing thepressure-equalizing valve 74 so as to now block further communicationbetween the flow line 67 and the borehole fluids exterior of the tool11. Simultaneously, the hydraulic fluid will also be admitted to thelower portion of the actuator 77 of the flow-line control valve 71. Byarranging the actuator 76 for the normally-open pressure-equalizingvalve 74 to operate somewhat quicker than the actuator 77 for thenormally-closed flow-line control valve 71, the second valve will bemomentarily retained in its closed position until the first valve hashad time to close. Then, once the pressure-equalizing valve 74 closes,as the hydraulic fluid enters the lower portion of the actuator 77 ofthe flow-line control valve 71, the latter valve will be opened ashydraulic fluid is exhausted from the upper portion of its actuatorthrough a typical check valve 136 and a branch return line 137 coupledto the retract line 92.

It will be appreciated, therefore, that with the tool 11 in the positiondepicted in FIGS. 6A and 6B, the flow line 67 is now isolated from theborehole fluids and is in communication with the isolated portion of theearth formation 13 by way of the flexible conduit 68. It will berecalled from the preceding discussion of FIG. 4 that the fluid volumesin the branch flow line 80 as well as the portion of the main flow line67 between the flowline control valve 71 and the sample-chamber controlvalves 72 and 73 were previously expanded by the upward movement of thedisplacement piston 82 in the reduced-volume chamber 81. Thus, uponopening of the flow-line control valve 71, the isolated portion of theearth formation 13 will be communicated with the reduced-pressure spacerepresented by the previouslyisolated portions of the flow line 67 andthe branch conduit 80.

Of particular interest to the present invention, it should be furthernoted that should the formation 13 be relatively unconsolidated, therearward movement of the valve member 52 in cooperation with the forwardmovement of the fluid-admitting member 42 will allow only those looseformation materials displaced by the advancement of the fluid-admittingmember into the formation to enter the fluid-admitting member. This isto say, the fluid-admitting member 42 can advance into the formation 13only by displacing loose formation materials; and, since the spaceopened within the forward end of the fluid-admitting member by therearward displacement of the valve member 52 is the only place intowhich the loose formation materials can enter, further erosion of theformation materials will be halted once the fluid-admitting member hasbeen filled with loose materials as shown at 138 in FIG. 63. On theother hand, should a formation interval which is being tested berelatively well-compacted, the advancement of the fluid-admitting member42 will be relatively slight with its nose making little or nopenetration into the isolated earth formation. It will, of course, beappreciated that the nose of the fluid-admitting member 42 will be urgedoutwardly with sufficient force to at least penetrate the mudcake whichtypically lines the borehole walls adjacent to permeable earthformations. In this situation, however, the forward movement of thefluid-admitting member 42 will be unrelated to the rearward movement ofthe valve member 52 as it progressively uncovers the filtering member66. In either case, the sudden opening of the valve 52 will cause theplug of mudcake in the nose of the fluid-admitting member 42 to bepulled to the rear of the filter 66. The significance of these actionswill be subsequently explained.

As best seen in FIGS. 6A and 68, therefore, should there be anyproducible connate fluids in the isolated earth formation 13, theformation pressure will be effective for displacing these connate fluidsby way ofthe new and improved fluid-admitting means 10 into the flowline until such time that the lower portion of the flow line 67 and thebranch conduit 80 are filled and pressure equilibrium is established inthe entire flow line. By arranging a typical pressure-measuringtransducer, as at 140 (or, if desired, one or more other suitable typesof property-measuring transducers) in the flow line 67, one or moremeasurements representative of the characteristics of the connate fluidsand the formation 13 may be transmitted to the surface by a conductor141 and either indicated or, if desired, recorded on the recordingapparatus 17 (FIG. 1). In any event, the pressure measurements providedby the transducer 140 will, of course, permit the operator at thesurface to readily determine the formation pressure as well as to obtainone or more indications representative of the potential producingcapability of the formation 13. The various techniques for analyzingformation pressures are well known in the art and are, therefore, of nosignificance to understanding the present invention.

The measurements provided by the pressure transducer 140 at this timewill indicate whether the sealing pad 39 has, in fact, establishedcomplete sealing engagement with the earth formation 13 inasmuch as theexpected formation pressures will be recognizably lower than thehydrostatic pressure of the borehole fluids at the particular depthwhich the tool 11 is then situated. This ability to determine theeffectiveness of the sealing engagement will, of course, allow theoperator to retract the tool-anchoring member 20 and the sealing pad 39without having to unwittingly or needlessly continue the remainder ofthe complete operating sequence.

Assuming, however, that the pressure measurements provided by thepressure transducer show that the sealing pad 39 is firmly seated, theoperator may leave the formation-testing tool 11 in the position shownin FIGS. 6A and 63 as long as it is desired to observe as well as torecord the pressure measurements. As a result, the operator candetermine such things as the time required for the formation pressure toreach equilibrium as well as the rate of any pressure increase andthereby obtain valuable information indicative of variouscharacteristics of the earth formation 13 such as permeability andporosity. Moreover, with the illustrated embodiment of the tool 11, theoperator can readily determine if collection of a fluid sample iswarranted.

Before'eontinuing with a description of a complete testing operation itis believed appropriate to now consider the details of the presentinvention. The significance of the present invention will be bestunderstood with the performance of the new and improved fluidadmittingmeans 10 while obtaining a pressure measurement or fluid sample iscompared to the performance of prior-art formation testers withconventional filtering members. Typically, these prior-art filtermembers have been an elongated tubular member having. only a pluralityof narrow slits ofa uniform width which are disposed eitherlongitudinally along the tubular member or circumferentially around themember. U.S. Pat. No. 3,352,361 is an example of this previous practice.Alternatively, either porous members or finelymeshed screens of aconventional design have often been employed as described, for example,in U.S. Pat. No. 3,653,436. In any case, these prior-art tools haveemployed conventional filters having only uniformly sized filteropenings which are customarily sized as dictated by the particular sizeof loose formation particles which were expected to be encounteredduring a given operation.

It has been found, however, that when these prior-art filters are usedin soft formations, the pressure drop across the filtering element andthe accumulated formation particles will often become so excessive thata fluid sample simply cannot be obtained in a reasonable period of time.This is easily understood when a priorart testing tool such as shown inU.S. Pat. No. 3,653,436 is considered. As shown in FIG. 5 of that patent, fluids entering the nose of the sampling tube will be divided intoa number of fluid paths, with the shortest path being through the firstopening in the filter screen at the forward end of the sampling tube andthe longest path theoretically being through the sampling tube and outthe rearwardmost opening in the screen. In actuality, however, it hasbeen found that by virtue of the additional flow resistance imposed bythe tightlypacked column of finely-divided sand particles which will betrapped in the sampling tube, most, if not all, of the flow will bethrough the forwardmost openings in the filter screen. Thus, since, atbest, little or none of the flow will be through the rearward portionsof the screen, the overall flow rate will be drastically curtailed. Itshould be noted in passing, however, that in this situation, it isunlikely that the filter screen will be entirely plugged by, mudcakesince any mudcake initially entering the sampling tube is typicallyconcentrated at the rearward end of the tube and held there by thecolumn of sand since the mudcake particles are too large to pass throughfilter openings small enough to retain the sand particles. Experiencehas shown, however, that if the screen openings are slightly oversizedso that some sand grains will pass through the front openings, it is notat all uncommon for the sand to gradually erode the filter screen to thepoint that the screen is no longer effective. Thus, enlargement of theopenings to improve the flow rate will often result in rapid failure ofthe filter.

A more-serious problem is encountered, however, when a prior-art testingtool equipped with a conventional filter having very narrow slits isused to test a fairly competent or hard formation. In this situation,the usual result will be that the mudcake entering the sampling tubewill swirl around inside of the tube so that the internal or inlet faceof the filter screen will be quickly coated with the mudcake particlesthereby plugging the narrow filter openings. Heretofore, the onlypractical solution to this problem has been to use a screen with thelargest-possible openings that will hopefully still trap any looseformation materials which might be encountered. This obviously poses aproblem where formations composed of different degrees of hardness orcompetency are expected to be encountered during a multi-formationtesting operation such as is capable of being performed by the tool 11.Thus, if the filter openings are too large, sand will easily passthrough the filter screen when unconsolidated formations are tested. Onthe other hand, if the screen openings are too small, they will beeasily plugged by mudcake when hard formations are tested.

The practice of the methods of the present invention as well as the newand improved fluid-admitting means 10 avoids these several problems,however. Accordingly, as best seen in FIGS. 7 and 8, somewhatsimplifiedenlarged views are respectively shown of the new and improvedfluid-admitting means 10 at successive moments during the initiation ofa test of an incompetent earth formation, as at 13, which is primarilycomposed of extremely-fine particles of sand and the like. At the timeillustrated in FIG. 7, the various elements of the tool 11 have justbeen placed in their re spective positions as previously described byreference to FIGS. 6A and 6B. Thus, as previously discussed, uponadvancement of the fluid-admitting member 42 into the formation 13, theplug of mudcake 142 in the nose of the fluid-admitting member will beimpelled from the wall of the borehole 12 into the tubular member andits interior will be quickly filled with the loose formation materials138 that are correspondingly displaced into the fluid-admitting memberas it penetrates the formation.

As illustrated in FIG. 7, since the plug of mudcake 142 from the wall ofthe borehole 12 enters the fluidadmitting member 42 ahead of theinrushing formation materials 138, the mudcake will, of course, becarried to the rear of the tubular member as the valve member 52 ismoved to the rear of the filter member 66. However, instead of themudcake plug 142 coming to rest at the rear of the fluid-admittingmember 42 as has been the case with prior-art testing tools, the mudcakewill be capable of passing through the rearward slits 143 in the screen66. As will be subsequently described, however, the uniquely-arrangedfilter member 66 will trap the incoming sand particles so as to quicklyform a compacted column of these particles, as at 138.

To accomplish this, the filtering member 66 of the new and improvedfluid-admitting means 10 is selectively arranged so that at least therearwardmost filter openings or slits 143 are individually wider thanthe forwardmost openings or slits I44. If desired, theintermediately-located slits, as at 145, in the filtering member 66 alsocan be selectively sized to have a width somewhat less than the width ofthe rear slits I43 but slightly greater than the width of the forwardslits 144. This can, of course, be accomplished in different manners.For example, as shown by the preferred embodiment in FIGS. 7 and 8, theseveral filter openings are respectively arranged ascircumferentially-oriented elongated slits which are disposed inmultiple sets of two or three slits around the filter member 66, withthe several sets being distributed along almost the full length of thefilter member and respectively sized or incrementally graduated so thatthe rearwardmost slits, as at 143, are selectively wider than theforwardmost slits, as at 144. Alternatively, as shown in somewhat of anexaggerated form in FIG. 9, the filter member 66' could also be arrangedwith a plurality of elongated longitudinally-oriented slits spaceduniformly around the circumference of the filter member and tapered orprogressively graduated so as to have relatively-wide rear portions, asat 143, and relatively-narrow forward portions, as at 144*.

In either manner, during the practice of the methods of the presentinvention with the tool 11, mudcake, as at 142, which enters thefluid-admitting member 42 at the very outset of the testing operation iscapable of freely passing through the enlarged rearward slits 143 (orthe rear slit portions 143') and on into the flow line 67. Therefore, asshown in both FIGS. 7 and 9, by virtue of the new and improved methodsand apparatus of the present invention mudcake, as at 142, iseffectively purged from the interiors of the fluid-admitting member 42and the filtering member 66 so as to eliminate this mudcake as a sourceof possibly-plugging materials such as frequently occurs during thetesting of fairlycompetent formations, as at 14, in FIG. 9.

It must be recognized, however, that the presence of the enlarged filteropenings 143 (or the rearward slit portions 143) presents apotentially-serious problem where the formation, as at 13, issubstantially composed of unconsolidated fine materials such as the sandparticles 138. As previously discussed, unless the flow of such fineparticles into the fluid-admitting member 42 is quickly halted, theisolated wall portion of the unconsolidated formation, as at 13, will berapidly eroded away to the extent that the sealing pad 39 will no longerbe in sealing engagement with the wall of the borehole 12.

Accordingly, as a further significant aspect of the inventive concept ofthe new and improved methods and apparatus disclosed here, the rearfilter openings 143 (or the rearward slit portions 143') are selectivelysized so that once a significant number of the rapidlyinrushing sandparticles, as at 138, have entered the fluid-admitting member 42, thesefine particles will be capable of quickly and reliably bridging the rearopenings. This bridging action should, of course, occur by the time thefluid-admitting member 42 is fully extended. Once this bridging occurs,the compacted column of collected sand grains 138 will thereafter serveas an auxiliary filtering medium which will at least significantlyreduce, if not completely block, further fluid flow through at least therearwardmost filter slits 143 (as well as the rearward slit portions143). It will, of course, be appreciated that the narrower forward slits144 (as well as the forward slit portions 144) must be selectively sizedto positively retain sand particles of a given size under evenrelatively-high flow rates. On the other hand, although the widerrearward slits 143 (as well as the rear slit portions 143) aresufficiently larger than the forward slits 144 (as well as the frontslit portions 144') so as to easily pass mudcake particles at high flowrates, these rearward slits and slit portions cannot exceed a particularwidth which will reliably create rapid bridging of the entrapped sandparticles 138 once the increasing pressure drop across the column ofsand particles has effected a significant reduction in the flow rate offluids passing through these rear slits and slit portions.

To understand the operation of the new and improved fluid-admittingmeans of the present invention, a description of fluid theory as itrelates to the fluid-admitting means is believed in order. With the newand improved fluid-admitting means 10 in the position illustrated inFIG. 7, for example, it will be recognized that if the formation 13contains producible connate fluids, these fluids can enter the flow lineonly as fast as these fluids can pass through the fluid-admitting member42 and the filtering member 66. The total flow rate of these fluidswill, as a matter of course, be directly governed by the degree of flowrestriction presented by the column of entrapped formation particles I38and the filtering member 66.

It will be recognized, of course, that the pressure differential betweenany arbitrary point, as at 146, in the nose of the tubular member 42 andthe annular space 64 will be a constant for a given flow situation; andthat this overall pressure differential will be a function of the totalflow rate, the total restriction presented by the filter member 66, andthe total restriction of the column of entrapped formation particles138. Fluids entering the fluid-admitting member 42 must, of course,divide into a number of flow paths, as at 147-149, in order to passthrough the various openings 143-145 along the full length of thefiltering member 66 before the fluids recombine in the annular space 64.Thus, for there to be any fluid flow along the rearwardmost flow path147, for example, the total pressure drop of fluids flowing along thatpath between the point 146 and the chamber 64 cannot exceed the overallavailable pressure differential then existing between these twolocations.

The total pressure along any one of the several flow paths 147-149 is,of course, the total of each of the partial pressure drops along thatpath. Thus, for the longest flow path 147, the total pressure drop willbe the summation of the pressure drop through the entire columnar lengthof the entrapped particles 138 and the pressure drop through therearwardmost openings or slits 143 in the filter member 66. On the otherhand, the pressure drop along the shortest path 149 will be the total ofthe drop through only. the first few of the trapped particles 138 andthe drop through the forwardmost openings or slits 144 in the filtermember 66. This, of course, means that for any given testing situationwith an unconsolidated formation, the entering fluids will be inherentlydivided proportionally along the several flow paths 147-149, with theflow rate along any one of these flow paths being a function of thecombined incremental pressure drops at that flow rate along the lengthof the path through the column ofsand grains 138 and whichever one ofthe several slits 143-145 that portion passes through. Since the overallpressure drop along each of these paths 147-149 will be the same forthat particular situation, the net result will be that a majorpercentage of the total flow will be through the forward slits 144, asignificant percentage of the flow will perhaps be through theintermediate slits 145, and, at best. only a minor percentage of theflow will be through the enlarged rearward slits 143.

This division of the flow along the several flow paths 147-149 is,therefore, a critical aspect of the present invention. As previouslymentioned, the rearward slits 143 must, on the one hand, be large enoughto reliably pass particles of mudcake at least when a competentformation is being tested and, on the other hand, be small enough toreliably effect a quick bridging of sand grains across these slits whenan unconsolidated form ation is instead being tested. This criticallimitation is best achieved by sizing the rearward slits 143 so that,with whatever overall pressure differentials that may be reasonablyexperienced during the testing of an unconsolidated formation, thecompacted column' of sand grains 138 will present such a substantialflow restriction that only minimal flows can pass along the flow path147 and pass through the enlarged slits without disrupting the bridge ofsand grains formed across those slits.

Thus, by deliberately restricting the flow path 147 through theserearward slits 143 (or the enlarged rear slit portions 143') in thismanner, it can be reliably assured that the flow of fluids therethroughwill be well below the critical flow rate which would-preclude eitherthe formation or the maintenance of bridges of the sand grains in thecompacted column 138 across the rearward slits. In other words, for agiven width of the rearward slits 143 and for a given overall availablepressure differential between the point 146 and the annular space 64,adetermination can be easily made (either by empirical testing orcalculations) of the maximum allowable flow rate of fluids which can bepassed safely through these enlarged slits before sand grains of a sizeordinarily retained by the forward slits 144 will no longer bridgeacross the rearward slits. Knowing this, it is, of course, simple tothen determine length of the column of sand grains 138 required toprovide a flow restriction sufficient to keep the flow rate along thelong flow path 147 well below the maximum allowable flow rate which willbe reliably supported by the sand particles bridging the rearward slits143.

It will, of course, be appreciated that the same criteria can be appliedto designing intermediately-located slits, as at 145, to have anintermediate width if this is desired. There will naturally be aproportional reduction in the restriction provided by the compactedcolumn of sand grains 138 since the flow path 148 goes through aproportionately-shorter length of the column. Thus, since this lesserrestriction will result in a proportionately-greater flow rate along theintermediate flow path 148 for a given overall pressure differential,the intermediate slits must be somewhat narrower than the rearward slits143 to maintain a bridge of sand particles across the intermediateslits. This degree of refinement is believed unnecessary, however. Byway of example, with the filter member 66 arranged generally as depictedin FIG. 7, it has been found that three rows of the rear slits 143 eachwith a width of 0.0l8-inch and eight rows of slits each with a width of0.0l-inch for the intermediate and forward slits 145 and 144,respectively, will enable the sample-admitting means 10 to effectivelyfunction in most, if not all, testing situations and still providemuch-greater flow rates in finely-divided formation materials than werepossible with prior-art filters.

Although FIG. 9 depicts an alternative embodiment of apparatus arrangedin accordance with the principles of the present invention, it will benoted that in the illustrated situation, the formation, as at 14, ismore competent than the formation 13. As a result, the fluidadmittingmember 42 has not been able to move forwardly to its furtherest-possibleextended position. This has, therefore, resulted in substantially fewersand particles entering the flud-admitting member so that a much-higheroverall flow rate is possible than would have occurred if thefluid-admitting member 42 had been filled with such particles. In thissituation, it is, of course, quite possible that some, if not all, ofany entering sand particles will simply flow on through the rearportions 143 of the tapered slits. The same thing would, of course,occur with the filter member 66. Thus, should there be only a shortcolumn of the sand particles (or none at all) captured in the filtermember 66', the larger available flow area defined by the rear slitportions 143 (or the rear slits 143) will simply result in greater flowrates than would otherwise be possible with conventional filters. Theimportant thing to note here is that in the testing ofarelatively-competent formation, the unique design of the filter 66' (aswell as the filter 66) with the enlarged rear slit portions 143' (or therear slits 143) will assure the passage of most, if not all, of themudcake which typically lines the wall of the borehole 12 where ittraverses a permeable formation, as at 14. Thus, regardless of which oneof the filters 66 or 66 is being employed with the new and improvedfluid-admitting means 10, there will be little or no mudcake retained inthe filter member which would otherwise be capable of plugging thefilter. By virtue of the enlarged filter openings 143 (or 143), themudcake will instead be free to quickly pass on into the flow line 67 sothat the interior of the filter 66 (or 66') will remain fully open. Itwill, of course, be recognized that since few if any formation particleswill be dislodged from the wall of the borehole 12 in this situation,there will be no occasion requiring bridging of the particles over therearward openings 143 (or 143) to prevent continued erosion of theisolated portion of the formation 14.

Now that the methods and apparatus of the present invention have beenfully set out, it is believed necessary only to quickly summarize thebalance of the complete operating cycle of the tool 10. Accordingly,referring again to FIGS. 6A and 68, it will be appreciated that once theseveral components of the tool 11 and the control system 16 have movedto their respective positions shown in these figures, the hydraulicpressure will again rise until such time that the set line pressureswitch 96 operates to halt the hydraulic pump 84. Inasmuch as thepressure switch 96 has a selected operating range, in the typicalsituation the pump 84 will be halted shortly after thepressure-equalizing valve 74 closes and the flow-line control valve 71opens. At this point in the operating cycle of the tool 11, once asufficient number of pressure measurements have been obtained aspreviously described, a decision can be made whether it is advisable toobtain one or more samples of the producible connate fluids present inthe earth formation 13. If such samples are not desired, the operatorcan simply operate the control switches 24 and 25 for retracting thetool-anchoring member 20 as well as the sealing pad 39 without furtherado. This freedom of action is, of course, made possible by virtue ofthe flexibility of operation of the new and improved fluidadmittingmeans 10 and the assurance that connate fluids can reliably pass throughthe filter member 66.

On the other hand, should a fluid sample be desired. the controlswitches 24 and 25 (FIG. 1) are advanced to their next or so-calledsample positions 29 to open, for example, a solenoid valve 150 (FIG, 3B)for coupling pressured hydraulic fluid from the highpressure section 116of the set line 91 to the piston actuator 78 of the sample-chambercontrol valve 72. This will, of course, be effective for opening thecontrol valve 72 to admit connate fluids through the flow line 67 andthe branch conduit 69 into the sample chamber 22. If desired, a chamberselection" switch 151 (FIG. 1) in the surface portion of the controlsystem 16 could also be moved from its first sample position 152 to itsso-called second sample" position 153 (FIG. 1) to energize a solenoidvalve 154 (FIG. 3B) for opening the sample-chamber control valve 73 toalso admit connate fluids into the other sample chamber 23. In eithercase, one or more samples of the connate fluids which are present in theisolated earth formation 13 can be selectively obtained by the testingtool 11.

Upon moving the control switches 24 and 25 to their so-calledsample-trapping positions 30, the pump 84 will again be restarted. Oncethe pump 84 has reached operating speed, it will commence to operatemuch in the same manner as previously described and the hydraulicpressure in the output line 87 will again begin rising with momentaryhalts at various intermediate pressure levels.

Accordingly, when the control switches 24 and 25 have been placed intheir sample trapping positions 30 (FIG. 1 the solenoid valve 94 (FIGS.3A and 38) will open to admit hydraulic fluid into the retract line 92.By means of the electrical conductor 98a, however, the pressure switch98 is enabled and the pressure switch 97 is disabled so that in thisposition of the control switches 24 and 25 the maximum operatingpressure which the pump 84 can initially reach is limited to thepressure at the operating pressure level determined by the pressureswitch 98. Thus, by arranging the hydraulic control valve 103 to open inresponse to a hydraulic pressure corresponding to this predeterminedpressure level, hydraulic fluid in the high-pressure section 116 of theset line 91 will be returned to the reservoir 86 by means of the returnline 89. As the hydraulic fluid in the high-pressure section 116 returnsto the reservoir 86, the pressure in this portion of the set line 9lwill be rapidly decreased to close the hydraulic control valve once thepressure in the line is insufficient to hold the valve open. Once thehydraulic control valve 100 closes, the pressure remaining in thelow-pressure section 113 ofthe set line 91 will remain at a reducedpressure which is nevertheless effective for retaining thetool-anchoring member 20 and the sealing pad 39 fully extended.

As hydraulic fluid is discharged from the lower portion of the pistonactuator 78 by way of the still-open solenoid valve 150 and fluid fromthe retract line 92 enters the upper portion of the actuator by way of abranch line 155, the sample-chamber control valve 72 will close to trapthe sample of connate fluids which is then present in the sample chamber22. Similarly, should a fluid sample have also been collected in theother sample chamber 23. the sample-chamber control valve 73 can also bereadily closed by operating the switch 151 (FIG. 1) to reopen thesolenoid valve 154. Closure of the sample-chamber control valve 72 (aswell as the valve 73) will, of course, be effective for trapping anyfluid samples collected in one or the other or both of the samplechambers 22 and 23.

Once the sample-chamber control valve 72 (and, if

necessary, the control valve 73) has been reclosed, the

control switches 24 and 25 are moved to their next or so-called retreat"switching positions 31 for initiating the simultaneous retraction ofthetool-anchoring member and the sealing pad 39. In this final positionof the control switch 25, the pressure switch 98 is again renderedinoperative and the pressure switch 97 is enabled so as to now permitthe hydraulic pump 84 to be operated at full rated capacity forattaining hydraulic pressures greater than the first intermediateoperating level in the retract cycle. Once the pressure switch 98 hasagain been disabled, the pressure switch 97 will now function to operatethe pump 84 so that the pressure will now quickly rise until it reachesthe next operating level.

At this point, hydraulic fluid will'be supplied through the retract line92 and the branch hydraulic line 135 for reopening thepressure-equalizing control valve 74 to readmit borehole fluids into theflow line 67. Opening of the pressure-equalizing valve 74 will admitborehole fluids into the isolated space defined by the sealing pad 39 soas to equalize the pressure differential existing across the pad beforeit is retracted. Hydraulic fluiddisplaced from the upper portion of thepiston actuator 76 of the pressure-equalizing valve 74 will bedischarged through a typical relief valve 156 which is arranged to openonly in response to pressures equal or greater than that of this presentoperating level. The hydraulic fluid displaced from the piston actuator76 through the relief valve 156 will be returned to the reservoir 86 byway of the branch hydraulic line 130, the high-pressure section 116 ofthe set line 91, the still-open hydraulic control valve 103, and thereturn line 89.

When the hydraulic pressure in the output line 87 has either reached thenext operating level or, if desired, a still-higher level, pressuredhydraulic fluid in the retract line 92 will reopen the hydraulic controlvalve 102 to communicate the low-pressure section 113 of the set line 91with the reservoir 86. When this occurs, hydraulic fluid in the retractline will be admitted to the retract sides of the several pistonactuators 21 and 41. Similarly, the pressured hydraulic fluid will alsobe admitted into the annular space 51 in front of the enlargeddiameterpiston portion 49 for retracting the fluidadmitting member 42 as well asinto the annular space 62 for returning the valve member 52 to itsforward position. The hydraulic fluid exhausted from the several pistonactuators 21 and 41 as well as the piston chambers 50 and 63 will bereturned directly to the reservoir 86 by way of the low-pressure section113 of the set line 91 and the hydraulic control valve 102. This actionwill, of course, retract the tool-anchoring member 20 as well as thesealing pad 39 against the tool body 19 to permit the tool 11 either tobe repositioned in the borehole 12 or to be returned to the surface ifno further testing is desired.

It should be noted that although there is an operating pressure appliedto the upper portion of the piston actuator 77 for the flow-line controlvalve 71 at the time that the pressure-equalizing valve 74 is reopened,a normally-closed relief valve 157 which is paralleled with the checkvalve 136 is held in a closed position until the increasing hydraulicpressure developed by the pump 84 exceeds the operating level used toretract the tool-anchoring member 20 and the sealing pad 39. At thispoint in the operating sequence of the new and improved tool 11, theflow-line control valve 71 will be reclosed.

The pump 84 will, of course, continue to operate until such time thatthe hydraulic pressure in the output line 87 reaches the upper limitdetermined by the setting of the pressure switch 97. At some convenienttime thereafter, the control switches 24 and 25 are again returned totheir initial or off positions 26 for halting further operation of thepump motor as well as reopening the solenoid valve 99 to againcommunicate the retract line 92 with the fluid reservoir 86. Thiscompletes the operating cycle of the illustrated embodiment of the tool1 1.

Accordingly, it will be appreciated that the new and improvedfluid-admitting means 10 of the present in vention enable theformation-testing tool, such as that shown herein at 11, to be operatedfor testing any type of formation which may be reasonably expected to beencountered during a formation-testing operation. By providing a filtermember with selectively-larger filter openings at the rear of themember, it is assured that a buildup of formation particles in thesample member will not block the flow of connate fluids through at leastthe rear portion of the fluid-admitting member into the portions of thefluid-admitting means. Thus, with the new and improved fluid-admittingmeans described herein, tests may now be conducted in various types offormations without experiencing either unduly-reduced flow rates where agiven formation is composed of exceptionally-fine, unconsolidated sandparticles or plugging of the filtering means with mudcake or the likewhere a relatively-complete formation is encountered.

While only one method and particular embodiments of apparatus of thepresent invention have been shown and described, it is apparent thatchanges and modifications may be made without departing from thisinvention in its broader aspects; and, therefore, the aim in theappended claims is to cover all such changes and modifications as fallwithin the true spirit and scope of the present invention.

What is claimed is:

l. A method for obtaining samples of connate fluids from earthformations traversed by a borehole and having mudcake lining theboreholes wall adjacent thereto and comprising the steps of:

packing-off a portion of said borehole wall adjacent to earth formationstherebeyond for isolating said wall portion and said earth formationsfrom fluids in said borehole;

inducting connate fluids from said formations through filtering meanshaving paralleled filter passages with at least a'first one of saidfilter passages being of an enlarged size sufficient to pass particlesof mudcake and at least a second one of said filter passages being of areduced size sufficient to retain loose formation materials forinitially drawing mudcake removed from said isolated wall portion onthrough said first filter passage; and, thereafter, inducting additionalconnate fluids from said earth formations through said filtering meansfor depositing loosened formation particles in a permeable bridge overat least said first filter passage so as to at least reduce thecontinued flow of connate fluids therethrough and increase the continuedflow of connate fluids through said second filter passage.

2. The method of claim 1 wherein said first and second filter passagesare respectively arranged as separated elongated slits in said filteringmeans.

3. The method of claim 1 wherein said first and second filter passagesare arranged as a single elongated slit in said filtering means havingan enlarged portion defining said first filter passage and a reducedportion defining said second filter passage.

4. A method for obtaining samples of connate fluids from earthformations traversed by a borehole and having mudcake lining theborehole wall adjacent thereto and comprising the steps of:

packing-off a borehole wall adjacent to earth formations therebeyond forisolating a portion of said wall and said earth formations from fluidsin said borehole;

inducting connate fluids from said isolated wall portion and earthformations through filtering means having at least one enlarged filterpassage sized to pass particles of said mudcake and in parallel flowrelationship with at least one reduced filter passage sized to retainloose formation particles for initially directing at least a substantialportion of said mudcake particles on through said enlarged filterpassage; and

whenever loose formation particles are eroded from said isolated wallportion, collecting such loosened particles with said filtering meansfor reducing the flow of connate fluids through said enlarged filterpassage sufficiently to build a bridge of such loosened particles acrosssaid enlarged filter passage and directing at least a major portion ofthe flow of connate fluids through said reduced filter passage to retainany subsequently-loosened formation particles.

5. The method of claim 4 further including the step of:

measuring at least one property of connate fluids passing through saidfiltering means for determining one or more characteristics of saidearth formations.

6. The method of claim 4 further including the step of:

collecting at least one sample of connate fluids passing through saidfiltering means.

7. The method of claim 4 further including the steps of:

obtaining at least one measurement of the pressure of connate fluidspassing through said filtering means for determining one or morecharacteristics of said earth formations; and, thereafter,

collecting at least one sample of connate fluids passing through saidfiltering means.

8. Formation-testing apparatus adapted for suspension in a boreholehaving mudcake lining the walls thereof adjacent to earth formationscontaining producible connate fluids and comprising:

a body having a fluid passage adapted to receive connate fluids;

fluid-admitting means on said body including a fluid entry coupled tosaid fluid passage and adapted to be engaged with a borehole wall forisolating a surface thereof from borehole fluids;

means selectively operable for positioning said fluid admitting meansagainst a borehole wall to place said fluid entry in communication withearth formations beyond the isolated wall surface of said borehole wall;and

fluid-filtering means cooperatively arranged between said fluid passageand said fluid entry for initially passing mudcake particles displacedfrom said isolated wall surface on through said filtering means intosaid fluid passage and operable thereafter whenever loose formationparticles of a size smaller than such mudcake particles are eroded fromsaid isolated wall surface for collecting such loosened smallerformation particles as connate fluids pass on through said filteringmeans into said fluid passage.

9. The formation-testing apparatus of claim 8 wherein saidfluid-filtering means include:

a filter member having at least one reduced filter passage sized toretain loose formation particles, and at least one enlarged filterpassage in parallel flow relationship with said reduced filter passagesized to pass mudcake particles and cooperatively arranged forcollecting loosened formation particles in a bridge across said enlargedfilter passage for reducing the flow of connate fluids therethrough sothat at least a major portion of connate fluids will then be directedthrough said reduced filter passage.

l0. The formation-testing apparatus of claim 9 wherein said filterpassages are arranged as a single elongated slit having an enlarged endportion for defining said enlarged filter passage and a reduced endportion for defining said reduced filter passage.

11. The formation-testing apparatus of claim 9 further including:

means selectively. operable after disengagement of said fluid-admittingmeansfrom said borehole wall for displacing from said flud entry anyloosened formation materials previously collected as a bridge acrosssaid enlarged filter passage.

12. The formation-testing apparatus of claim 9 further including:

pressure-measuring means coupled to said fluid passage and adapted forproviding at least one measurement representative of the pressure ofconnate fluids in said fluid passage.

13. The formation-testing apparatus of claim 9 further including:

sample-collecting means on said body and selectively operable forobtaining a sample of connate fluids in said fluid passage.

14. The formation-testing apparatus of claim 9 further including:

pressure-measuring means coupled to said fluid passage and adapted forproviding at least one measurement representative of the pressure ofconnate fluids in said fluid passage; and

1. A method for obtaining samples of connate fluids from earth formations traversed by a borehole and having mudcake lining the boreholes wall adjacent thereto and comprising the steps of: packing-off a portion of said borehole wall adjacent to earth formations therebeyond for isolating said wall portion and said earth formations from fluids in said borehole; inducting connate fluids from said formations through filtering means having paralleled filter passages with at least a first one of said filter passages being of an enlarged size sufficient to pass particles of mudcake and at least a second one of said filter passages being of a reduced size sufficient to retain loose formation materials for initially drawing mudcake removed from said isolated wall portion on through said first filter passage; and, thereafter, inducting additional connate fluids from said earth formations through said filtering means for depositing loosened formation particles in a permeable bridge over at least said first filter passage so as to at least reduce the continued flow of connate fluids therethrough and increase the continued flow of connate fluids through said second filter passage.
 2. The method of claim 1 wherein said first and second filter passages are respectively arranged as separated elongated slits in said filtering means.
 3. The method of claim 1 wherein said first and second filter passages are arranged as a single elongated slit in said filtEring means having an enlarged portion defining said first filter passage and a reduced portion defining said second filter passage.
 4. A method for obtaining samples of connate fluids from earth formations traversed by a borehole and having mudcake lining the borehole wall adjacent thereto and comprising the steps of: packing-off a borehole wall adjacent to earth formations therebeyond for isolating a portion of said wall and said earth formations from fluids in said borehole; inducting connate fluids from said isolated wall portion and earth formations through filtering means having at least one enlarged filter passage sized to pass particles of said mudcake and in parallel flow relationship with at least one reduced filter passage sized to retain loose formation particles for initially directing at least a substantial portion of said mudcake particles on through said enlarged filter passage; and whenever loose formation particles are eroded from said isolated wall portion, collecting such loosened particles with said filtering means for reducing the flow of connate fluids through said enlarged filter passage sufficiently to build a bridge of such loosened particles across said enlarged filter passage and directing at least a major portion of the flow of connate fluids through said reduced filter passage to retain any subsequently-loosened formation particles.
 5. The method of claim 4 further including the step of: measuring at least one property of connate fluids passing through said filtering means for determining one or more characteristics of said earth formations.
 6. The method of claim 4 further including the step of: collecting at least one sample of connate fluids passing through said filtering means.
 7. The method of claim 4 further including the steps of: obtaining at least one measurement of the pressure of connate fluids passing through said filtering means for determining one or more characteristics of said earth formations; and, thereafter, collecting at least one sample of connate fluids passing through said filtering means.
 8. Formation-testing apparatus adapted for suspension in a borehole having mudcake lining the walls thereof adjacent to earth formations containing producible connate fluids and comprising: a body having a fluid passage adapted to receive connate fluids; fluid-admitting means on said body including a fluid entry coupled to said fluid passage and adapted to be engaged with a borehole wall for isolating a surface thereof from borehole fluids; means selectively operable for positioning said fluid-admitting means against a borehole wall to place said fluid entry in communication with earth formations beyond the isolated wall surface of said borehole wall; and fluid-filtering means cooperatively arranged between said fluid passage and said fluid entry for initially passing mudcake particles displaced from said isolated wall surface on through said filtering means into said fluid passage and operable thereafter whenever loose formation particles of a size smaller than such mudcake particles are eroded from said isolated wall surface for collecting such loosened smaller formation particles as connate fluids pass on through said filtering means into said fluid passage.
 9. The formation-testing apparatus of claim 8 wherein said fluid-filtering means include: a filter member having at least one reduced filter passage sized to retain loose formation particles, and at least one enlarged filter passage in parallel flow relationship with said reduced filter passage sized to pass mudcake particles and cooperatively arranged for collecting loosened formation particles in a bridge across said enlarged filter passage for reducing the flow of connate fluids therethrough so that at least a major portion of connate fluids will then be directed through said reduced filter passage.
 10. The formation-testing apparatus of claim 9 wherein said filter passages are arrangEd as a single elongated slit having an enlarged end portion for defining said enlarged filter passage and a reduced end portion for defining said reduced filter passage.
 11. The formation-testing apparatus of claim 9 further including: means selectively operable after disengagement of said fluid-admitting means from said borehole wall for displacing from said flud entry any loosened formation materials previously collected as a bridge across said enlarged filter passage.
 12. The formation-testing apparatus of claim 9 further including: pressure-measuring means coupled to said fluid passage and adapted for providing at least one measurement representative of the pressure of connate fluids in said fluid passage.
 13. The formation-testing apparatus of claim 9 further including: sample-collecting means on said body and selectively operable for obtaining a sample of connate fluids in said fluid passage.
 14. The formation-testing apparatus of claim 9 further including: pressure-measuring means coupled to said fluid passage and adapted for providing at least one measurement representative of the pressure of connate fluids in said fluid passage; and sample-collecting means on said body and selectively operable for obtaining a sample of connate fluids in said fluid passage.
 15. The formation-testing apparatus of claim 14 further including: means selectively operable after disengagement of said fluid-admitting means from said borehole wall for displacing from said fluid entry any loosened formation materials previously collected as a bridge across said enlarged filter passage.
 16. Formation-testing apparatus adapted for suspension in a borehole having mudcake lining the walls thereof adjacent to earth formations containing producible connate fluids and comprising: a body having a fluid passage adapted to receive connate fluids; fluid-admitting means on said body and including a fluid-sampling member having an inlet passage adapted to be engaged with a borehole wall for isolating a surface thereof from borehole fluids and an outlet passage downstream of said inlet passage and coupled to said fluid passage; means selectively operable for positioning said fluid-sampling member against a borehole wall to place said inlet passage into communication with earth formations beyond the isolated wall surface of said borehole; and filtering means cooperatively arranged on said fluid-sampling member for intercommunicating said inlet and outlet passages and including means defining a chamber adapted for collecting loose formation particles entering said inlet passage, means defining at least one enlarged filter passage between said chamber and said outlet passage and selectively sized to initially pass mudcake particles entering said particle-collecting chamber into said outlet passage as well as to subsequently develop a bridge of loosened formation particles across said enlarged filter passage whenever such entering formation particles are collected in said particle-collecting chamber, and means defining at least one reduced filter passage upstream of said enlarged filter passage and selectively sized to retain such loosened formation particles without halting the flow of connate fluids between said inlet and outlet passages.
 17. The formation-testing apparatus of claim 16 wherein said fluid-sampling member is tubular with a forward portion thereof defining said inlet passage, said outlet passage includes an inwardly-opening fluid chamber formed in an intermediate portion of said fluid-sampling member, and said filtering means include a filter member covering the entrance of said inwardly-opening fluid chamber and having a forward portion with said reduced filter passage therein and a rearward portion with said enlarged filter passage therein and defining at least part of a side wall of said particle-collecting chamber.
 18. The formation-testing apparatus of claim 17 wherein said filter passages are separated from oNe another.
 19. The formation-testing apparatus of claim 17 wherein said filter passage are joined to one another by an intermediate filter passage having a width less than that of said enlarged filter passage.
 20. The formation-testing apparatus of claim 17 wherein said filter passages are joined to one another by an intermediate filter passage having a width less than that of said enlarged filter passage and greater than that of said reduced filter passage.
 21. The formation-testing apparatus of claim 17 further including: a piston member coaxially arranged in said fluid-sampling chamber for movement between a retracted position in the rearward portion thereof to define the rear wall of said particle-collecting chamber and a normal extended position in said forward portion of said fluid-sampling member to block said inlet passage; and means coupled to said piston member and selectively operable for moving said piston member to its retracted position following engagement of said fluid-sampling member with a borehole wall to admit connate fluids into said inlet passage and for returning said piston member to its extended position following disengagement of said fluid-sampling member from a borehole wall to expel any lossened formation materials previously collected as a bridge across said enlarged filter passage from said fluid-sampling member.
 22. Formation-testing apparatus adapted for suspension in a borehole having mudcake lining the walls thereof adjacent to earth formations containing producible connate fluids and comprising: a body having a first fluid passage adapted to receive connate fluids; fluid-admitting means on said body and including a fluid-sampling member having a tubular forward portion adapted to be engaged with a borehole wall for isolating a surface thereof from borehole fluids; means selectively operable for positioning said fluid-sampling member against a borehole wall to establish communication with earth formations therebeyond; first means adapted for limiting the entrance of loose formation particles into said first fluid passage including a second fluid passage in said fluid-sampling member and coupled to said first fluid passage, and filtering means cooperatively arranged on a wall of said fluid-sampling member for controllably communicating said second fluid passage with said tubular forward portion and including enlarged filter passage means selectively sized to initially pass mudcake particles entering said fluid-sampling member as well as to subsequently develop a bridge of loosened formation particles across said enlarged filter passage means whenever such formation particles enter said fluid-sampling member and reduced filter passage means upstream of said enlarged filter passage means and selectively sized for retaining loosened formation particles within said fluid-sampling member as filtered connate fliuds pass through said filtering means into said second fluid passage; second means adapted for controlling the entrance of particles into said fluid-sampling member and including a piston member coaxially arranged in said fluid-sampling member for movement past said filtering means between an advanced position within said tubular forward portion ahead of said reduced filter passage means blocking communication into said fluid-sampling member and a retracted position to the rear of said enlarged filter passage means for uncovering said filtering means and defining a space adjacent to said enlarged filter passage means for collecting loosened formation particles entering said tubular forward portion; and piston-control cooperatively arranged for selectively moving said piston member back and forth between its said advanced and retracted positions.
 23. The formation-testing apparatus of claim 22 wherein said second fluid passage includes an annular chamber formed around an intermediate interior wall portion of said fluid-sampling member between said advanced and retracted positionS of said piston member; and said filtering means include a tubular filter member coaxially mounted in said fluid-sampling member around said annular chamber and sized for passage of said piston member as it moves between its said positions.
 24. The formation-testing apparatus of claim 23 wherein said filter passage means are comprised of a plurality of longitudinally-oriented slits circumferentially spaced around said filter member and respectively having enlarged rearward portions defining said enlarged filter passage means and reduced forward portions defining said reduced filter passage means.
 25. The formation-testing apparatus of claim 24 wherein said longitudinally-oriented slits are symmetrically tapered between said enlarged rearward portions and said reduced forward portions.
 26. The formation-testing apparatus of claim 23 wherein said filter passage means are comprised of a plurality of circumferentially-oriented slits spaced longitudinally along said filter member with at least the rearwardmost ones of said slits being wider than at least the forwardmost ones of said slits for respectively defining said enlarged filter passage means and said reduced filter passage means.
 27. The formation-testing apparatus of claim 26 wherein all of said slits ahead of said wider rearwardmost slits are of an equal width which is narrower than the width of said wider rearwardmost slits.
 28. The formation-testing apparatus of claim 22 further including: sealing means cooperatively arranged on said fluid-admitting means around said tubular forward portion and adapted for packing-off a borehole wall around said tubular forward portion.
 29. The formation-testing apparatus of claim 22 further including: means cooperatively mounting said fluid-sampling member on said body for movement relative thereto between a laterally-extended position and a retracted position; and control means cooperatively arranged for selectively moving said fluid-sampling member back and forth between its said extended and retracted positions.
 30. The formation-testing apparatus of claim 22 further including: sample-receiving means on said body and adapted for receiving filtered connate fluids entering said first fluid passage; and control means cooperatively arranged for selectively coupling said sample-receiving means to said first fluid passage.
 31. The formation-testing apparatus of claim 22 further including: pressure-monitoring means for providing indications at the surface indicative of the pressure of connate fluids in said first fluid passage.
 32. The formation-testing apparatus of claim 22 further including: a wall-engaging member movably mounted on said body on the opposite side thereof from said forward portion of said fluid-sampling member; and piston means cooperatively coupled to said wall-engaging member for moving said wall-engaging member between an extended wall-engaging position and a retracted position against said opposite body side to respectively engage and disengage said fluid-admitting means with and from a borehole wall.
 33. The formation-testing apparatus of claim 22 further including: means cooperatively coupling said fluid-sampling member to said body for movement relative to one side thereof between a laterally-extended position and a retracted position; and piston means cooperatively coupled to said fluid-sampling member for moving said fluid-sampling member between its said laterally-extended position and its said retracted position to respectively engage and disengage said fluid-sampling member with and from a borehole wall.
 34. The formation-testing apparatus of claim 33 further including: a wall-engaging member movably mounted on said body on the opposite side thereof; and piston means cooperatively coupled to said wall-engaging position and a retracted position against said opposite body side upon movement of said fluid-sampling member between its said positions. 